1. Field of the Invention
The present invention relates to an ultraviolet-curable and alkali-developing type photo solder resist ink which can be used to manufacture printed circuit boards.
2. Disclosure of the Prior Art
Recently, dry films or liquid type photo solder resist inks having excellent resolution and dimensional accuracy have been widely used to form a solder resist pattern in a manufacturing process of a printed circuit board with high wiring density. For example, Japanese Patent Publication [KOKOKU] No. 56-40329 discloses a composition comprising an ultraviolet-curable type resin obtained by a reaction of an unsaturated monocarboxylic acid with an epoxy resin and an addition of a polybasic acid anhydride to the epoxy resin. In addition, Japanese Patent Early Publication [KOKAI] No. 61-243869 discloses a liquid type photo solder resist ink containing thermosetting components of an epoxy compound, a diluent, a photopolymerization initiator, and an alkali-developing and ultraviolet-curable type resin obtained by an addition of a polybasic acid anhydride to a novolac-type epoxy acrylate.
By using these resist inks, a solder resist pattern can be formed on a substrate for a printed circuit board according to the following method. That is, the resist ink is applied to the substrate having a conductor pattern to obtain a resist ink layer thereon by a dipping method, spray method, screen printing, and so on. Next, as a precuring step, an organic solvent contained in the resist ink layer is volatilized at a temperature of 60.degree. C. to 120.degree. C. to obtain a resist ink film. After a mask having a desired pattern is mounted on the resist ink film, ultraviolet is radiated to the resist ink film through the mask by using a light source such as a high-pressure mercury-vapor lamp, xenon lamp, chemical lamp, ultrahigh-pressure mercury vapor lamp, metal halide lamp, and so on. In a developing step after the ultraviolet radiation, unexposed portions of the resist ink film are dissolved by an alkaline solution to obtain a patterned resist ink film. Then, the patterned resist ink film is heated at 120.degree. C to 180.degree. C. for about 30 minutes to cure the epoxy compound in the resist ink film, so that strength, hardness and chemical resistance of the resist ink film are improved. This step is known as a post-baking step. In this method, the precuring step is very important to obtain the resist ink film having non-tackiness. The non-tackiness of the resist ink film improves a removability of the mask from the resist ink film after the exposing step, and prevents a contamination of the mask.
However, when the resist ink contains an epoxy compound, there is a problem that a thermosetting phenomenon is caused at the precuring step by a reaction between the epoxy compound and carboxylic group of the ultraviolet-curable resin. This brings about a deterioration of the developing property or the resolution of the resist ink. In particular, when the epoxy compound is a solvent-dissolving type epoxy compound, the precuring step is acceptable only under a limited gentle condition with respect to temperature and time, for example, less than 20 minutes at 80.degree. C., and in a particular case less than 30 minutes at 80.degree. C. An acceptable condition range of the precuring step is called as a precuring acceptable width.
In such a gentle condition of the precuring step, since a diluent often remains in the resist ink film, there are problems that the removability of the mask from the resist ink film lowers and the contamination of the mask easily occurs. This means that a tacky-dry property of the resist ink film is poor. When the resist ink film is excellent in the tacky-dry property, no tackiness is felt when fingers lightly touch a surface of the resist ink film.
In the actual manufacturing process of printed circuit boards, the resist ink film is often stored for several days after the precuring step. In such a case, there is a tendency that the developing property and the resolution of the resist inks described above are deteriorated because the reaction of the epoxy compound with carboxylic group of the ultraviolet-curable resin gradually proceeds at a room temperature during the storing period.
For solving the above problems, Japanese Patent Early Publication [KOKAI] Nos. 1-141904, 3-250012, and 4-217252 disclose ultraviolet-curable and alkali-developing type photo solder resist inks. In those references, a fine powder of triglycidyl isocyanurate or the like having a sparing solubility to the diluent is used as the epoxy compound. The fine powder is mixed with an ultraviolet-curable resin obtained by an addition of an acid anhydride to a bisphenol A-type epoxy acrylate or a novolac-type epoxy acrylate to provide the ultraviolet-curable and alkali-developing type photo solder resist inks.
In case of using these resist inks, since the powder-like epoxy compound is not uniformly dispersed as a solid state in the resist ink film at the precuring step, contacts of the epoxy compound with the carboxylic group of the ultraviolet-curable resin decrease, so that the curing reaction is difficult to proceed. In addition, since the ultraviolet-curable component exists at a high concentration in the resist ink film except for the dispersed portions of the powder-like epoxy compound after the precuring step, a decrease in the light-sensitivity of the resist ink film can be prevented. Moreover, when the resist ink film is heated at a higher temperature than a melting point of the epoxy compound at the post-baking step, the strength, hardness and chemical resistance of the resist ink film can be improved. Due to these advantages, the powder-like epoxy compound having the sparing solubility to the diluent may be preferably used.
By the way, after the precuring step, the thermosetting property of the resist ink film is influenced by the particle size of the powder-like epoxy compound. Even when the powder-like epoxy compound is melted at the post-baking step, it is difficult to obtain a uniform mixing state of the epoxy compound and the ultraviolet-curable resin in the resist ink film. Consequently, definite boundaries between the epoxy compound and the ultraviolet-curable resin components occur. The definite boundaries prevent a curing reaction therebetween at the post-baking step to cause disadvantages with respect to the heat resistance and the electrical corrosion resistance of the resist ink film.
For solving this inconvenience, it is proposed to make an extremely fine powder of the epoxy compound by various physically pulverizing methods. But, there is a limit in the particle size of the fine powder obtained by the physically pulverizing methods. Japanese Patent Early Publication [KOKAI] No. 6-95384 discloses a photo solder resist ink comprising an ultraviolet-curable resin and triglycidyl isocyanurate as the epoxy compound which is a solid state at a room temperature and has the sparing solubility to the diluent. It indicates that making the extremely-fine powder of the epoxy compound and utilizing a particular dispersing method of the powder-like epoxy compound are important to improve the light-sensitivity of the resist ink at the exposing step and enhance the thermosetting reaction at the post-baking step. In an example of this reference, it is described that a sufficient effect can be not obtained by simply pulverizing triglycidyl isocyanurate by a roll-mill and so on.
Thus, the precuring acceptable width can be improved by using the extremely fine powder of the epoxy compound having the sparing solubility to the diluent. However, there is a limit as to the improvement of the heat resistance and the electrical corrosion resistance of the resist ink film.